Much of the recent work dealing with the control of breathing has neglected events occurring during expiration because convenient methods have not been available. In preliminary studies I have developed a new method to determine the threshold for expiratory-inspiratory (E-I) phase switching in anesthetized cats. This method uses tonic vagal stimulation and measures the threshold by quantitating how much the frequency of the tonic stimulation must be reduced to trigger E-I phase switching. This new method permits the study of the response of the E-I phase switch to hypercapnia and hypoxia. Comparing the response of the threshold to traditional respiratory parameters (expiratory duration, inspiratory duration, and the rate of rise and peak of both the diaphragmatic and abdominal electromyogram) will help clarify relationships between events occurring during inspiration and expiration. It will also define the relationship between the E-I phase switch and expiratory output (the latter, presumably, a reflection of central expiratory excitation, CEE). More importantly this new method permits the study of temporal processing by the E-I phase switch to controlled afferent inputs, thereby determining the characteristics of integration and accommodation. This should narrow the possible mechanisms by which the respiratory rhythm may originate. The mechanism of long-term adaptation to tonic afferent stimulation will also be examined. Neuro transmitters and their antagonists will be administered. This could lead to an understanding of the pathogenesis of the dyspnea and the altered breathing patterns seen in those cardio-pulmonary disorders in which there is abnormal afferent activity. This in turn may form the basis of a rational pharmacologic approach to therapy.